An OLED Display can be classified according to its driving method, passive-matrix (PMOLED) and active-matrix (AMOLED). AMOLED uses TFT (Thin Film Transistor) with a capacitor for storing data signals that can control OLED levels of brightness.
The manufacturing procedure of PMOLED is simpler in comparison and is less costly of the two; however, it is limited in its size (<5 inches) because of its driving mode and a lower resolution display application. In order to produce an OLED display with higher resolution and larger size, utilizing active-matrix driving is necessary. The so-called AMOLED uses TFT (Thin Film Transistor) with a capacitor for storing data signals, so that pixels can maintain their brightness after line scanning; on the other hand, pixels of passive matrix driving only light up when the scan line selects them. Therefore, with active matrix driving, the brightness of OLED is not necessarily ultra-bright, resulting in longer lifetime, higher efficiency and higher resolution. Naturally, TFT-OLED with active matrix driving is suitable for display application of higher resolution and excellent picture due to the unique qualities of OLED.
LTPS (Low Temperature Poly-Silicon) and a-Si (amorphous Silicon) are both technologies of TFT integrating on glass substrate. The obvious differences are electric characteristics and complexity of process. Although LTPS-TFT possesses higher carrier mobility and higher mobility means more current can be supplied, the process is much more complex. However, the process of a-Si TFT is simpler and more mature, except for low carrier mobility. Therefore, a-Si process has better competitive advantages in cost.
As mobility of LTPS-TFT is up to 100˜200 cm2 /V-sec currently, TFT-OLED driving IC and data IC can be LTPS processed; however, due to limitations of LTPS processing capability, properties of each TFT element vary. The most pressing problem of AMOLED is how to reduce the impact of uneven LTPS-TFT characteristics. Such an issue requires an immediate solution for follow-up development and applications since images with erroneous gray scales show up on OLED panels and seriously damage image uniformity.
U.S. Pat. No. 6,229,506 discloses an Active Matrix Light Emitting Diode Pixel Structure And Concomitant Method. A 4T2C (4 TFTs and 2 capacitors) pixel circuit is proposed as shown in FIG. 1. An Auto-Zero mechanism is applied to compensate for threshold voltage differences of TFT elements to improve the uniformity of images. Driving sequences of control signals include Auto-Zero Phase 210, Load Data Phase 220 and Illuminate Phase 230. Refer to FIG. 2 for the sequences of control signals.
Transistors T3 and T4 are off and transistor T2 is on prior to Auto-Zero Phase 210. The current passing through OLED 160 at this moment is current of the previous frame and controlled by Vsg of transistor T1 (voltage difference between source and gate; i.e., voltage difference of both ends of Cs).
After entering the Auto-Zero Phase 210, transistor T4 is on and then transistor T3 is on, too so that drain and gate of transistor T1 can connect as a diode. As transistor T2 is off, gate voltage of transistor T1 will increase, which equals to Vdd minus threshold voltage (Vth) of transistor T1. That is to say, the voltage difference stored at both ends of capacitor Cs is the threshold voltage of transistor T1. After placing transistor T3 off, threshold voltage (Vth) of transistor T1 can be stored into capacitor Cs and Auto-Zero Phase 210 is completed.
On Load Data Phase 220, when the voltage difference of data line 110 is ΔV, it couples to the gate of transistor T1 through transistor T4 and capacitor Cc. Thus, voltage difference stored at both ends of capacitor Cs will be ΔV×[Cc/(Cc+Cs)] adding Vth that is stored in capacitor Cs previously. That is, Vsg of transistor T1 includes Vth of transistor T1, which makes output current of transistor T1 relate to voltage change (ΔV) of data line 110 and capacity of capacitors Cc and Cs, instead of being affected by Vth of transistor T1 in every pixel.
Lastly, when Illuminate Phase 230 begins, transistor T4 is off and transistor T2 is on. Output current of transistor T1 at the present frame will flow through OLED 160 to illuminate.
Though this 4T2C pixel circuit may compensate for the threshold voltage (Vth) differences of transistor elements in each pixel and improve integral uniformity of images; however, other control lines like Auto-Zero Line 130 and Illuminate Line 140 are required in addition to data line 110, scan Line 120 and supply line (Vdd) 150. Capacitor Cs has to record all threshold voltages and part of the data voltages loaded. Besides, a capacitance coupling approach is used to load data, which not only makes the driving method more complicated, but also increases manufacturing costs when a non-standard data driving IC is required.
To solve the same problem, Philips also published a thesis with the subject of ┌ A Comparison of Pixel Circuits for Active Matrix Polymer/Organic LED Displays┘. One 4T2C pixel circuit is presented in the thesis as FIG. 3 shows. It skillfully changes the location of connecting two capacitors in the pixel circuit of the U.S. Pat. No. 6,229,506 (FIG. 1) to solve the defects of complexity and impracticability. However, control lines like Auto-Zero Line 330 and Illuminate Line 340 are also required in addition to data line 310, scan line 320 and supply line (Vdd) 350, just like those in U.S. Pat. No. 6,229,506.
The sequences of driving control signals are the same as those in the U.S. Pat. No. 6,229,506 since they consist of Auto-Zero Phase, Load Data Phase and Illuminate Phase.
On Auto-Zero Phase, Transistor T34 is off and then transistor T33 is on so that drain and gate of transistor T31 can be connected as a diode. As transistor T32 is off, gate voltage of transistor T31 will increase, which equals to Vdd minus threshold voltage (Vth) of transistor T31. That is to say, the sum voltage stored at capacitors C1 and C2 is the threshold voltage (Vth) of transistor T31. After placing transistor T33 off, Auto-Zero Phase is completed.
Data voltage is conducted through connection of transistor T34. Data voltage is stored in capacitor C1 and a certain proportion of Vth previously stored at both ends of capacitor C2 is still maintained, which equals to [C1/(C1+C2)]×Vth. Thus, the sum of capacitors C1 and C2 is (Vdd−Vdata+[C1/(C1+C2)]×Vth); i.e., Vsg of transistor T31 contains part of Vth of transistor T31, which may not only reduce the correlation between the output current and threshold voltage of transistor T31, but also compensate for part of the threshold voltage (Vth) difference resulting from processing factors.
The threshold voltage of transistor T31 in the thesis is memorized by two capacitors (C1 & C2). Part of the threshold voltage data stored in one of the capacitors will get lost while loading data voltage. Therefore, this approach can only make up for part of the threshold voltage difference resulting from processing.